Printers with which the invention, as applied to a sheet feeder, is useable, which may be for example of the electrophotographic, ink jet or laser jet type, are generally designed to print on a number of different sheet sizes. These sizes are standardized and include legal, letter, A3, A4, B5 and others. Known sheet feeders such as trays, containers or cassettes for printers are able to accommodate different sizes of sheets by including sheet size guides which are manually adjustable to fit a desired sheet size. Commonly, an operator knows the size of the sheets stored in a sheet feeder and appropriately formats the printer driver or copying machine for that size. A more desirable arrangement, however, would be to sense automatically the sheet size and generate an electrical signal which can be used to control a printing function related to the sheet size. For example, a signal which is representative of the size of sheets in a sheet feeder could be forwarded to a printer driver for the driver to determine whether the sheet size is appropriate for the image to be printed. Should the image not be compatible with the sensed sheet size, the printing can be altered and/or a signal sent to the operator to correct the matter. Another more intelligent option is for the signal to initiate an automatic scaling of an image to fit onto the sheet size which is in the sheet feeder.
U.S. Pat. No. 5,573,236 discloses a sheet storage tray which is adapted to detect the size of sheets in the tray. This adaptation comprises the provision of an optical reflective sensor and a continuously variably graduated scale associated with a sheet guide. Movement of the sheet guide in the tray causes the scale to be moved past the sensor. When the guide is located against a stack of sheets, the sensor generates a signal having a strength which is determined by the relative position of the scale. Thus the strength of the signal is representative of the position of the guide and thus the size of the sheets.
There are a number of problems with the arrangement disclosed in U.S. Pat. No. 5,573,236. First, it requires a specially prepared continuously variably graduated scale. Example scales which are disclosed are a grey scale which varies from black to white with levels of grey therebetween, a continuous colour pattern that varies in colour from end to end, a variably transparent scale and binary scales to give digital signal outputs. Second, as the sensor is movable relatively along the continuously variably graduated scale at a constant spacing, and as an "electrical output v. level of light sensed" characteristic curve for the reflective sensors concerned are not normally determined for such a scale and a constant measurement distance, and given such sensors can vary significantly in performance from sensor to sensor, a special calibration of each unit to determine a curve of output v. light for the particular sensor and the particular scale in that unit is required. Thus a relatively complex calibration on every unit is required and this adds to the expense of a unit. Also the scales are expensive to manufacture, and the performance of the prior art arrangements depend on the consistency with which the patterns or markings are made on such scales. They are also prone to degradation through contamination with dirt and dust etc. and thus lose accuracy, which problem is dealt in U.S. Pat. No. 5,573,236 by the provision of a wiper operatively associated with the sensor to clean the scale as it moves past the sensor. Some of these types of scales also require the use of special sensors to suit the scale in question, for example if the scale is a digital type scale.